A problem in the development of TTSs is the guarantee of a contour-sealing bond with the skin when wearing the product for periods of a few hours up to 7 days or more. Contour-sealing in this connection means that detachment of an outer edge of the TTS from the skin is avoided.
Effects on the wearing behaviour originate from the size of the TTS and from the administration site on the human body. The mechanical forces acting on the TTS (e.g. stretching, compression, torsion and shear forces), which can lead to detachment from the skin, substantially result from these two parameters.
In order to guarantee that a TTS adheres to the skin for comparatively long periods, an optimization of the adhesive layer facing the skin is necessary. Depending on the construction of the TTS, this layer can be active substance-containing and moreover can contain pharmaceutical auxiliaries which favour the dermal permeation of the active substance. In such cases, the adhesive behaviour of the layer is affected to a greater or lesser extent by the nature and amount of the active agents and auxiliaries contained. Possible further layers of the system may additionally change the state of the layer adhering on the skin side by absorption or release of substances.
A special role is played by the absorption of cutaneous excretions by the TTS. These comprise excretions of gaseous water (transpiration), of liquid water (sweat), of salts (sweat) and of lipids (sebaceous matter). The absorption of water, in particular, leads to a diminished cohesion and shear resistance of the internal structure, whereby the adhesive strength can become reduced even to detachment. Furthermore, considerable and unforeseen changes of the inner structure of the TTS with regard to cohesion, shear resistance and adhesive power may occur as a consequence of individual components being released to the site of application.
In summary, the establishment of optimum adhesive properties is an object mainly concerning the formulation of the adhesive layer and moreover depends strongly on the internal and external construction of the TTS. The variabilities associated therewith make the generalization of successful formulations largely impossible. For each new TTS, a more or less laborious optimization of the adhesive properties is therefore necessary.
Tests on TTSs which are in the development stage are advantageously performed under realistic conditions on humans or animals. Especially when performed on humans, such studies require, however, very high financial expenditure because of the fact that clinical studies frequently fall under the law on drugs, as well as because of the number of individuals required for making clear statistical statements. In addition, experience shows that optimization of adhesive properties constitutes a process that is to be repeated several times in the course of developing a mature product.
For the above reasons, testing on humans takes place only at a late stage of the development. The adhesive strength of TTSs is preferably included as a parameter in clinical studies on therapeutic efficacy and harmlessness. At the early stage of the development, subjective criteria of those persons involved in the development have hitherto played an important part. This development also includes manual testing of the adhesive layer using one's hands, as well as short-term wearing tests on human subjects.
Objectifiable test methods in great part originate from the development of technical adhesives or pressure-sensitive adhesives (cf. official test methods of AFERA; Association des Fabricants Eruopeens de Rubans Auto-adhesifs, Vitry sur Seine, France). In the case of simple mechanical load tests the pressure-sensitive adhesive formulation to be tested is initially applied to a layer-shaped carrier with which it forms a firm composite. This adhesive tape-like test product is then stuck on test surfaces, and subsequently peel forces, for example, which act at varying peel-off angles and speeds, are determined. Given a suitable experimental array it is also possible to determine the tendency of an adhesive compound to flow under mechanical load (cold flow) due to a lack of cohesion.
Testing may be performed within wide limits by variation of parameters such as the properties of test surfaces, as well as temperature and air humidity. The individual tests, however, physically reflect only fractions of the dynamic loading process on the skin. In particular, it is difficult or even impossible to simulate an exchange of substances between the system and the subject wearing it. The mechanical loads are one-sided and virtually always aim at an extreme load on the adhesive, which is not common on the skin.
In summary, these methods are primarily suitable as a means for quality control to quarantee the uniformity of a product and less for optimizing the properties thereof in the development stage.
There are also further methods available for a differentiated assessment of the adhesive properties which typically are much more complicated technologically. For example, by measuring the wetting angles between drops of various test liquids and the surface of a pressure-sensitive adhesive it is possible to obtain data on the surface characteristics of the adhesive. In this way it is possible to detect the energetic relationships and, in particular, the distribution of chemically polar and nonpolar interactions at the outside of the adhesive. Ideally, these relationships are the same as those existing at the surface to be adhered, i.e. the skin in the case of a TTS. By means of this process one can ascertain the physico-chemical affinity of an adhesive to an application surface; the mechanical properties, such as, for instance, the capacity of conforming rapidly to a rough surface, are, however, left entirely out of consideration.
The mechanical, internal properties of adhesives are particularly the subject of methods employed in rheology and in dynamic mechanical analysis (DMA). With the aid of rheometers it is possible to carry out examinations on thin layers of adhesive under action of torsion forces. The type of torsion load as well as the temperature and air humidity during the test can be varied within wide limits. In DMA, in addition to exposure to torsion forces, it is also possible to expose the test material to bending, stretching and compression forces.
Both methods are very well suited for testing the cohesion and shear strength, especially when involving a change in temperature. Furthermore, it is possible to make statements on the cold flow, as well as on elastic and plastic proportions in the deformation of an adhesive mass. The methods are efficient but technically complicated. Nevertheless, they only take into account the internal properties of an adhesive, while the properties of the outer surfaces, as well as the actual tack, are not recorded. An exchange of substances with the environment, as takes place in the case of TTSs worn on the skin, can not be realized employed today's equipment.
Summarizing, there exists a broad range of test methods which, however, always permit statements to be made only on relatively narrow partial aspects of adhesion on the skin. The gathering of the data takes place under strongly abstracted conditions which aim at ascertaining individual physico-chemical characteristic values in their purest form.
There is a lack of realistic test methods and models which make it possible to simulate the wearing of TTSs on the skin in a less abstracted manner and representing the problems entailed as comprehensively as possible. Consequently, there is a demand to realize such a model without great technical effort and in a manner enabling wide-range use for routine testing.